From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6E8B727A477; Tue, 28 Jul 2026 01:13:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785201239; cv=none; b=EbpXdzUlfP1jzG1bT2cFnfi5qHotXmHZ2T4HVmiGoMMxtfn5iggXKiYjoYWOppKIQ3hr7NtSpp6t2UAwJF37q0iH7ubGWQOQm32QX3lX6QE7kNyAGuvDYSQX1w1sISQsY//rKIjSLq99Z5bJCBBmWdaNCYmPzbWt3PvmUmYwNEI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785201239; c=relaxed/simple; bh=4oHtG3v0tOUMa1Mw8i4F4wz1N31phh39K/DCtgAuMKk=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=YluQu0N4V+bNdYaVJwYtCsneHWjKQLWQPUOjh/Or8GW71mE2u1UvtroVmlt65+qUywgOWecU83J/lTdmA4yq9D555KyTataTmn5G0nNuetZASkNhvr4tJv10Ts8gGbCL68ISoB5t31zRTuo7F2FEXMkQuFcXJUZY9eU7H2AN0pY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=BGilTH8z; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="BGilTH8z" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3A54C1F000E9; Tue, 28 Jul 2026 01:13:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux-foundation.org; s=korg; t=1785201236; bh=NIXmI5Wrz5QQpJEi32OhaAP/sLdLGCY4PKg0EE41yN8=; h=Date:From:To:Cc:Subject:In-Reply-To:References; b=BGilTH8z74mcP66Zopm/CHbt5aJ75n75odHbFQLxDOFWNGetS4+SiUnn4vUx0L+76 JVjfXfLudFZ0BA2PEXQ/0dEPXvaxLAQSig19TXbSbMnbhFB+qjo1O4AJOIBEog4qqe NhzwvhxaKABySW72eumF9j3++S7m2h5rcT4wlUnE= Date: Mon, 27 Jul 2026 18:13:55 -0700 From: Andrew Morton To: pratmal@google.com Cc: vbabka@kernel.org, david@kernel.org, sj@kernel.org, corbet@lwn.net, skhan@linuxfoundation.org, anshuman.khandual@arm.com, gthelen@google.com, surenb@google.com, mhocko@suse.com, jackmanb@google.com, hannes@cmpxchg.org, ziy@nvidia.com, ljs@kernel.org, liam@infradead.org, rppt@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Link Lin Subject: Re: [PATCH v3] mm/page_reporting: Add page_reporting_delay_ms module parameter Message-Id: <20260727181355.481ebbe3b56425a760583f39@linux-foundation.org> In-Reply-To: <20260727230545.262579-1-pratmal@google.com> References: <20260727230545.262579-1-pratmal@google.com> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Mon, 27 Jul 2026 23:05:45 +0000 pratmal@google.com wrote: > From: Pratyush Mallick > > Currently, the free page reporting uses a hardcoded delay of > (2 HZ) between reporting intervals. While this is a reasonable > default, it lacks the flexibility to adapt to varying guest workloads. > > A low delay allows aggressive memory reclamation, returning unused > pages to the host as quickly as possible. However, during spiky > allocation/free churn, this immediate reporting can lead to a severe > performance penalty (nested page faults) as the guest re-allocates memory > that the host has just unmapped. In these scenarios, there is benefit > from increasing the delay to batch free pages over a longer window, > absorbing the churn without hypercall and re-fault overhead. > > This patch exposes the delay as a module parameter: > /sys/module/page_reporting/parameters/page_reporting_delay_ms, measured > in milliseconds and defaults to 2000ms. Of course we'd prefer some sort of self-tuning so the kernel automatically avoids the situation. But the user's expectation that reporting occurs at a fixed frequency messes up that concept. > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt lgtm. It conflicts with Link Lin's "mm/page_reporting: use system_freezable_wq to fix UAF during suspend". Easy resolution: Documentation/admin-guide/kernel-parameters.txt | 6 ++++ mm/page_reporting.c | 19 ++++++++------ 2 files changed, 17 insertions(+), 8 deletions(-) --- a/Documentation/admin-guide/kernel-parameters.txt~mm-page_reporting-add-page_reporting_delay_ms-module-parameter +++ a/Documentation/admin-guide/kernel-parameters.txt @@ -4813,6 +4813,12 @@ Kernel parameters Adjust the minimal page reporting order. The page reporting is disabled when it exceeds MAX_PAGE_ORDER. + page_reporting.page_reporting_delay_ms= + [KNL] Free page reporting delay in milliseconds + Format: + Adjust the delay in milliseconds between free page + reporting intervals. Default is 2000 (2 seconds). + panic= [KNL] Kernel behaviour on panic: delay timeout > 0: seconds before rebooting timeout = 0: wait forever --- a/mm/page_reporting.c~mm-page_reporting-add-page_reporting_delay_ms-module-parameter +++ a/mm/page_reporting.c @@ -48,7 +48,11 @@ MODULE_PARM_DESC(page_reporting_order, " */ EXPORT_SYMBOL_GPL(page_reporting_order); -#define PAGE_REPORTING_DELAY (2 * HZ) +static unsigned int page_reporting_delay_ms = 2 * MSEC_PER_SEC; +module_param(page_reporting_delay_ms, uint, 0644); +MODULE_PARM_DESC(page_reporting_delay_ms, + "Set page reporting delay in milliseconds"); + static struct page_reporting_dev_info __rcu *pr_dev_info __read_mostly; enum { @@ -77,12 +81,11 @@ __page_reporting_request(struct page_rep return; /* - * Delay the start of work to allow a sizable queue to build. For - * now we are limiting this to running no more than once every - * couple of seconds. + * Delay the start of work to allow a sizable queue to build. + * We limit this based on page_reporting_delay_ms. */ queue_delayed_work(system_freezable_wq, &prdev->work, - PAGE_REPORTING_DELAY); + msecs_to_jiffies(page_reporting_delay_ms)); } /* notify prdev of free page reporting request */ @@ -337,13 +340,13 @@ static void page_reporting_process(struc err_out: /* * If the state has reverted back to requested then there may be - * additional pages to be processed. We will defer for 2s to allow - * more pages to accumulate. + * additional pages to be processed. We will defer by + * page_reporting_delay_ms to allow more pages to accumulate. */ state = atomic_cmpxchg(&prdev->state, state, PAGE_REPORTING_IDLE); if (state == PAGE_REPORTING_REQUESTED) queue_delayed_work(system_freezable_wq, &prdev->work, - PAGE_REPORTING_DELAY); + msecs_to_jiffies(page_reporting_delay_ms)); } static DEFINE_MUTEX(page_reporting_mutex); _